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Raising Product Appeal Through Doming

(June 2004) posted on Tue Jun 15, 2004

Discover the tools and materials behind doming technology and how to incorporate the technology into your production workflow


By Ben P. Rosenfield

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We live in a three-dimensional world, which may explain why we often consider two-dimensional objects to be plain and uninteresting. Sometimes even the most vibrant, colorful decals, nameplates, tags, promotional items, and labels go without a second glance because they lack that certain something. The missing link between nice and noteworthy can be filled by doming these types of products. Doming is a finishing process that not only enhances the appearance of printed items by producing a three-dimensional look, but it also makes them more durable. And most importantly, doming creates an impression of value and quality in your products that quickly translates to greater profits for your business.

The mechanics of doming

The doming process relies primarily on three components: a dispensing system, a resin from which the dome or lens is formed, and a product with surface tension sufficient enough to accept and control the liquid resin during and after application. Essentially, the dispensing equipment draws or receives the doming resin from dedicated containers and then deposits the resin on the surface of a receptive product, usually a tag, decal, label, or nameplate that has previously been printed and die cut or knife cut. Provided that the surface energy of the material is high enough to allow the resin to wet its surface, the resin spreads until it reaches the cut edges of the part, forming a dome that encompasses the entire part (Figure 1). In this manner, a rectangular cut label forms a rectangular dome after resin is applied, a circular label a circular dome, and so on. As straightforward as this process may sound, successful doming hinges on your ability to control several key variables in the process.

Perhaps the greatest challenge in doming is figuring out how much resin to deposit onto a product's surface. If you dispense too much, the volume of the resin will overcome the product's surface tension and the liquid will flow over the edges of the label or tag. If you dispense too little, the resin may not reach the edges of the product's surface as it flows out.

So how much is enough? Experience and experimentation are the answer, according to W. Scott Anderson, president of Color-Dec Int'l., Ocean City, NJ. He says that 0.15-0.20 g/sq cm, or the 1 g/sq in. equation some domers use, are good starting points for estimating how much resin is needed for any given application. Anderson also notes that substrate composition and cutting method can affect the flow rate of the resins. He explains, "If it's metallized Mylar, it might flow differently than a vinyl-coated Mylar, and a plotter-cut label works better than a vertical-diecut label. The plotter-cut process creates more consistent edges."

Steel-rule diecuts or vertical diecuts, as Anderson puts it, may "fracture" the coating on the substrate. If you fracture this coating, the hygroscopic--moisture-seeking--nature of the catalyst component of polyurethane resin materials may actually sap the moisture from the substrate under the coating and bring that moisture into the resin. If that happens, you could be faced with clouding of the lens. However, domers have worked successfully with diecut materials for quite some time, so if you already have a die-cutting system in place, testing materials and cutting processes and consulting with dispensing-equipment and resin manufacturers should help make your existing tools suitable for use in preparing domed products.

Equipment

Dispensing equipment is available in numerous configurations and can accommodate products of varying shapes and sizes and job orders large and small. If doming is your desire, you have quite a few systems from which to choose. Before you begin, however, it might be helpful to review some basic features and terms associated with doming equipment. These descriptions apply primarily to machines that dispense two-part polyurethane resins, the most common type used today.

Meter-mix Also referred to as meter-mix-dispense, this means the dispensing machine meters (measures) the doming materials by volume and then mixes the components before depositing the resin onto the products.

Static mixer This refers to the dispensing machine's mixing unit, which is responsible for blending doming materials. A static mixer is often a simple, rigid, plastic tube that houses a series of helical elements. As both components of a two-part polyurethane resin enter the static mixer, they're sheared and blended together when they pass over each element. The result is a homogenous mixture ready for deposit. Static mixers are common in both entry-level manual and high-end automatic dispensing systems.

Dynamic mixer The dynamic mixer is similar to the static version in that it relies on the helical elements, but it also consists of a pneumatic motor that is integral to the dispensing head. This motor complements the static mixer's shearing and blending effect with rotary motion, which provides additional mixing.

Piston-metering systems Some dispensing machines rely on pistons to measure the resin and catalyst. The pistons' cylindrical housings are sized to conform to the volumetric ratio of the doming materials, 1:1 and 2:1 being the most common configurations. Upon actuation, the pistons push the precise amount of each component toward the mixer.

Gear-metering systems These are another means used to deliver predetermined amounts of doming components to the dispensing machine's mixer. The gears, controlled by a servo motor, are machined with precise volumetric spaces between each tooth, which makes them the metering system. Controlled amounts of doming materials are metered as they're captured by the teeth of the gears.

Scripting This is a specialty technology. Dispensing machines designed to handle scripting jobs are traditionally high-end units that are driven by dedicated microprocessors. Scripting machines are made to follow complex paths and are capable of multi-axis movements, including x, y, and z. Some advanced models also automatically address metering-system speed, rate of resin flow, and other operating factors.

So where do you begin when shopping for a doming system? You can make a minimal investment in the technology by purchasing a starter kit, which usually includes a hand-held dispensing gun, cartridges that are preloaded with doming-resins and catalysts, static-mixer tubes, and tools to help you remove bubbles and other imperfections you might encounter during resin application. Think of it as a marriage between the caulk gun and the two-part-adhesive syringe dispenser, both of which you might have on your home workbench. Starter kits allow you to experiment with doming different products, become familiar with the characteristics of doming resins, and get a good idea of what's involved in the process without taking a hammer to your company's piggy bank.

The next step up from the sample kits are manually driven dispensing machines for higher production volumes. Available in tabletop and floor-mounted configurations, these products usually come with fixed dispensing heads. While they're designed for doming lower volumes of products than automated machines, many are available with multi-nozzle manifolds that allow you to dome rows of multiple products at once (Figure 2). Some of these units may come with footswitches and other control options, but expect to index your sheets of printed products and move them along the production line by hand. According to Paul Nannig, vice president of sales at North Kingstown, RI-based Development Associates, machines in this category, while manually driven, offer "repeatability of shots, accuracy, and excellent production pace."

If you're looking to automate for higher levels of production, several op-tions exist. At one end of the spectrum are systems that are manually loaded and unloaded and provide automated x-axis-control of the dispensing head for doming repetitive products. At the other end are sophisticated machines capable of dispensing resins across x and y axes (Figure 3). Such units are usually computer-driven with virtually every feature semi or fully automated. Brad Smith, manager of N. Canton, OH-based Liquid Control Corp.'s doming business, claims that a majority of screen printers rely on machines that operate with limited automatic motion, or x-axis control. "If you have 100,000 decals to dome, you can line the sheets up on a table in front of you, and the machine's dispensing head will index straight across, shoot shots, and dome the decals," he says.

But if your printed products involve complex cut paths, such as cursive letters, and require more than a single shot of resin to create a dome, scripting technology is the way to go (Figure 4). Designed to dispense resins in multi-axis movements, scripting machines set themselves apart from other doming units in this respect. Control is the keyword in scripting technology. Scripting systems provide control of variables such as flow-rate modulation, point-to-point velocity management, and charting paths for resin dispensation.

According to Fabio Okada, product manager of Liquid Control Corp.'s automated dispensing work cells, "You're dispensing a bead and then targeting the center of whatever you're writing. You have to have good control on your motion system so it applies the correct amount of material at a constant speed."

Dynamic variables in scripting, such as those previously discussed, are grounded in computer technology. Scripting software prompts users to provide tolerances for these variables. At first, you may feel like a matchmaker setting up the equipment and printed products on a quirky blind date--after all, you're charged with instructing the dispensing system where to go, how fast to get there, how much resin to dispense when it arrives, and so on.

Some scripting systems allow users to scan products and plot the doming course on screen, some are compatible with CAD files and other digital file formats, and others may be configured so that the user can actually train the software by manually moving the dispense head around a plotted course. From there, trial and error help determine flow rate, but Todd Williams, sales manager at Newcastle, ME-based Sheepscot Machine Works, explains that, as a general rule of thumb, you'll want to reduce velocity of movement, as well as flow rate, as you pass from a wide area to a narrow one.

Consumable considerations

You face about as many choices in doming resins as in dispensing equipment. Epoxies, one-part UV-curable polyurethanes, and two-part polyurethanes are the most common; however, the two-part polyurethanes enjoy a greater share of the resin market. According to Leigh Davison, product manager at Chemque, Rexdale, ON, Canada, the industry started out with epoxies and then made the switch to polyurethanes based on overall performance. "Epoxy is falling out of favor," he says. "It has a certain place, but polyurethane is replacing it because of [polyurethane's] positive characteristics in terms of weathering and flexibility."

Resins, much like squeegee-blade materials, are shore tested for hardness. Softer, more flexible resins are best suited to applications in which curves, or more dramatic shapes, are present. Davison explains that if you try to apply a decal domed with a harder resin to a cylindrical surface area, such as a bicycle frame, that decal will most likely pull away. "The less flat the surface, the more flexible the resin needs to be," he says.

While they require no mixing and generally cure faster than two-part resins, epoxies and one-part UV-curable polyurethanes are prone to yellowing and other reactions after prolonged exposure to the elements. They are therefore best suited to disposable products or those intended for indoor, short-term, light-impact use.

"Two-part resin weathers better, shrinks less, and does not yellow or crack," Nannig says, "But there are, of course, varying degrees of that." Two-part polyurethane resin also has a memory of sorts, and its desire to return to the shape achieved in its cured state results in a self-healing effect.

Two-part polyurethanes are made from polyol and isocyanate components, which are the resin and catalyst, respectively. Common ratios used with two-part polyurethane resins include 1:1 and 2:1. The 1:1 ratio calls for equal parts of isocyanate and polyol, while the 2:1 ratio typically calls for two parts isocyanate and one part polyol. The 1:1 mixing ratio, according to Anderson, "is more of an entry-level measure, but some run 1:1 on high-production machines. Using two parts of the catalyst delivers better and more consistent mixing and is more consistent in gelling time and flow rate."

A major consideration when working with two-part resins is the hygroscopic property of the isocyanate. If given the chance, the isocyanate will draw moisture from the air, and in turn, contaminate itself. This can happen before the two components are mixed or after they're mixed and dispensed on the products. Typically, moisture contamination manifests itself in carbon-dioxide bubbles. Not only do these bubbles ruin the appearance of domed products, but they also can disrupt the component ratios within the dispensing equipment. Dirt, dust, and other particulates also are sources of contamination. If foreign objects find their way into your resins, whether before or after deposition, you will see the results in the domed products. On the other hand, Al DeSisto, director of technical sales for Deco-Coat Products, Cranston, RI, says epoxies are not affected by most shop conditions.

The solution to contamination problems may sound simple, but it's an ongoing effort. First, you'll need to set up an area dedicated to doming. The cleaner and drier, the better. Many manufacturers suggest setting up a clean room in which the air is filtered, the humidity controlled to 40-60% relative, and room temperature kept moderate. "You cannot expect to dome good parts without a clean room," Anderson says. "That means all air passes through HEPA filters and positive pressure is maintained ... and air locks and Tyvek suits are used, if possible."

Each resin, regardless of chemical composition, requires safe handling. De-Sisto says that even though doming resins are relatively safe, some people will have sensitivity to the materials. Anderson notes that doming-room ventilation is essential because most common catalysts are mercury-based. "You cannot concentrate too many domed labels in a fixed area because there is an emission from the labels that is not measurable in volume, but can lead to skin irritation." He also explains that keeping too many freshly domed products in the same area can slow down the curing process.

Maintaining equipment and consumables is another major concern. If unused portions of resins are properly stored, they will continue to perform as expected. Davison suggests inserting dessicant cartridge filters into the storage tanks' bleed valves. This ensures that any air entering the tanks is both dry and free of particulates. "When storing material, keep it off the floor and keep it at room temperature and in a dry environment," he says.

Become familiar with the anatomy of your dispensing machine and make its maintenance a daily ritual. Lubrication and system cleansing are a big part of this routine. Williams notes that some machines have shafts, fittings, pump cups, and other points that you should tend to without fail. And Smith explains that you can either flush the dispensing system with solvents at the end of the day or simply throw away some of the manifold components, such as tubing and nozzle outlets. "Once you take apart the manifold, whether to solvent flush or throw parts away, you also take the static mixer off because it's going to react [with the environment] and get hard overnight," he says.

According to Anderson, some new solvents, dubbed "safe cleaners," can be reclaimed up to 10 times with a simple filtration system, but he believes that methylene chloride, which is a known carcinogen, is best for effective system flushing. "You can clean a high-production machine with maybe half a cup of methylene chloride and not get a lot of fumes if you do it right," he says. "Other cleaners, you might need to run a quart. [Methylene chloride] is universally known to be a nasty solvent, but if you use it properly, you'll use such a small amount that it's not a real hazard to the environment." Whichever method you choose, keeping your equipment clean is a sure-fire way to see an ongoing return on what can be a sizable investment.

Dare to dome

The perceived and real value of doming benefits both the customer and the screen printer. The process breathes new life into everyday products and improves the screen printer's profit margin. For example, printers who decorate decals and sell them for five to ten cents a piece today can add six cents worth of doming resin to each and then sell each domed decal for 30 cents. Even though the price of admission into doming may at first seem steep, the value doming brings to your products can help set you apart from the competition and add a new dimension to your business.

Equipment and Resin Suppliers

Ashby Cross Co. 28 Parker St. Newburyport, MA 01950 978-463-0202 www.ashbycross.thomas register.com [Equipment]

Chemque Inc. 266 Humberline Dr. Rexdale, ON M9W 5X1 Canada 416-679-5676 800-268-6111 www.chemque.com [Resins]

Color-Dec Int'l. 3330 Simpson Ave. Ocean City, NJ 08226 800-430-9072 609-814-1484 www.color-dec.com [Equipment and resins]

Cytec 1405 Buffalo St. Olean, NY 14760 716-372-9650 www.conap.com [Resins]

Deco-Coat Products 21 Starline Way Cranston, RI 02921 888-376-9437 401-946-5564 www.deco-coat.com [Resins]

Demak SRL Strada del Cascinotto 10156, Torino Italy 39-011-223-9876 www.demak.it [Equipment]

Development Associates Inc. 300 Old Baptist Rd. N. Kingstown, RI 02852 401-884-1350 www.daius.com [Equipment and resins]

Fluid Research Corp. 1382 Bell Ave. Tustin, CA 92780 714-258-2350 www.fluidresearch.com [Equipment]

Liquid Control Corp. 8400 Port Jackson Ave. NW North Canton, OH 44720 330-494-1313 www.liquidcontrol.com [Equipment]

Mockridge Int'l./Mockridge Doming Systems Ltd. Cavendish St., Ashton under Lyne OL6 7QL United Kingdom 44-161-308-2331 www.mockridge.com [Equipment]

Rad-Cure Corp. 9 Audrey Pl. Fairfield, NJ 07004 973-808-1002 www.radcure.com [Resins]

Sealant Equipment and Engineering, Inc. 45677 Helm St. PO Box 701460 Plymouth, MI 48170 734-459-8600 www.sealantequipment.com [Equipment]

Sheepscot Machine Works 1130 U.S. Rt. 1 Newcastle, ME 04553 207-563-2299 www.sheepscotmachine.com [Equipment]

Unisect Resources Ltd. 9 Cork St. London W15 3LL United Kingdom www.unisect-resources.com [Resins]


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